The downlock pin for a 747 can be a "fishing pole" type. It is about 2m long with an actuating system to put a lock in place to stop the pin coming out. This prevents you from having to climb up into the bay to put the pin in. When installed it does rest on the trailing edge of the moveable doors, so it is possible that as the doors open the pin got caught on the doors and was pulled out, however, I would have thought that the lock should stop this from happening. I got told by someone who should know that the system had already been operated 5-6 times before it all went horribly wrong. There by the grace of God............